Add a 'make distcheck' test to CI

[jjhursey]

Occasionally OpenPMIx/PRRTE hit build system issues that only show up when making a distribution tarball. It has been suggested to me in the past that the community add a "special build" of OpenPMIx and PRRTE that checks for this scenario.

Suggested process:

• Build PRRTE and/or OpenPMix release tarball
• Extract the tarball - so we are only building with what is packaged
• Build it a few different ways. For example, with and without VPATH
• Run a few simple runtime tests, such as 'make check'

We have a "special builds" CI already active, so this would be adding a new directory and test script to the following directory in pmix-tests (CI will pick it up automatically once it is committed to master):

• https://github.com/openpmix/pmix-tests/tree/master/ci-builds